Speed control system for vehicles

ABSTRACT

In a vehicle speed control system, a preset offset distance is taken as a zero value of an inter-vehicle distance in a brake discriminant. When the current value of a corrected distance condition evaluation index is higher than the brake discriminant, a value on the brake discriminant is set as a target value of the corrected distance condition evaluation index. Deceleration control is started to decelerate the subject vehicle so that the deceleration of the subject vehicle becomes equal to the target deceleration of the subject vehicle computed based on the target relative speed corresponding to the set target value and the actual relative speed.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference thewhole contents of Japanese Patent Applications No. 2007-263683 filed onOct. 9, 2007, No. 2008-97536 filed on Apr. 3, 2008, and No. 2008-127664filed on May 14, 2008.

FIELD OF THE INVENTION

The present invention relates to a speed control system for vehicles.

BACKGROUND OF THE INVENTION

In conventional inter-vehicle distance control systems, the enhancementof response to inter-vehicle distance control between a subject vehicleand a preceding vehicle traveling ahead the subject vehicle and thesecurement of safety are achieved by taking the following measure.

For example, in the inter-vehicle distance control device described inPatent Document 1 (JP 2567548), when the vehicle speed of the subjectvehicle is high and when the inter-vehicle distance is short, the timeconstant of a filter for relative speed is reduced. When the vehiclespeed of the subject vehicle is low and when the inter-vehicle distanceis long, the time constant of a filter for relative speed is increased.However, this inter-vehicle distance control system controls the vehiclespeed of the subject vehicle based on the relative speed between thesubject vehicle and the preceding vehicle so that the inter-vehicledistance is kept constant. The feeling of deceleration at this time isnot always comfortable for the driver.

To cope with this, the drive assistance system described in PatentDocument 2 (US 2007/0021876, JP 2007-76632A) takes the followingmeasure. This drive assistance system takes, as a driver conditioncoefficient to be taken as a target (target driver conditioncoefficient), the driver condition coefficient in a state in which adriver in condition suitable for driving is performing driving operationso as to keep constant the inter-vehicle distance between the precedingvehicle and the subject vehicle. The drive assistance system computes arelative acceleration/deceleration between the subject vehicle and thepreceding vehicle to be taken as the target (target relativeacceleration/deceleration) based on the target driver conditioncoefficient and the present driver condition coefficient. The subjectvehicle is accelerated or decelerated based on this target relativeacceleration/deceleration.

The drive assistance system described in Patent Document 2 does not takethe speed of the preceding vehicle into account to determineacceleration/deceleration start timing. Therefore, the driver may not beable to feel the comfortableness of acceleration depending on thedriving scene. For this reason, it is proposed in U.S. patentapplication Ser. No. 12/151,998 filed on May 12, 2008 to determineacceleration/deceleration start timing by using an approximateexpression (brake discriminant KdBc) of a corrected distance conditionevaluation index KdBc with the speed of the preceding vehicle taken intoaccount so that comfortability can be thereby ensured. The distancecondition evaluation index is an evaluation index regarding approachingto and separating from the preceding vehicle.

In this brake discriminant KdBc, the inter-vehicle distance when thepreceding vehicle and the subject vehicle come closest to each other isensured by taking the following measure, as illustrated in FIG. 14. Whenthe inter-vehicle distance D between the preceding vehicle and thesubject vehicle is in a short distance range (actual 0 to Dq [m]), thediscriminant is substantially unchanged and the deceleration output isincreased by reducing an apparent target. For this reason, the target ofdeceleration output has an inflection point at some midpoint in theprocess of the inter-vehicle distance D being shortened. As a result,the deceleration output is varied and discomfort is brought to thedriver at the time of deceleration.

A brake system for vehicles is also proposed in U.S. patent applicationSer. No. 11/805,236, which uses an expression obtained by modifying thefollowing Expression 1 to control deceleration.|−Vr|=2.5×D ³×10^((|KdB|/10−8))  (Expression 1)

This expression is a formula for computation of distance conditionevaluation index, that is evaluation index of approaching/separatingrelative to the preceding vehicle. In this expression, Vr is targetrelative speed and KdB is distance condition (approaching/separating)evaluation index.

In this brake system, deceleration is controlled in the followingmanner. The initial value of distance condition evaluation index whenbrake operation is started is determined. The subsequent target value ofdistance condition evaluation index is increased with a constantgradient based on the slope (change rate) of distance conditionevaluation index at the start of the brake operation. The decelerationis controlled based on the target relative speed corresponding to thistarget value and the actual relative speed.

However, the deceleration control using the technique described in theabove patent application may not be matched with the feeling of thedriver for the following reason.

Assuming that the distance D at the start of control is D0 and thedistance condition evaluation index at the start of control be KdB0, thegradient of a tangent of Expression 1 at the start of decelerationcontrol is expressed by Expression 2:dKdB|dD=−30×log₁₀ e/D0=−13.03/D0  (Expression 2)

According to Expression 2, an intercept of the tangent is expressed byExpression 3:y-intercept=KdB0+13.03/D0×D0=KdB0+13.03  (Expression 3)

According to the gradient and y-intercept expressed by Expressions 2 and3 and the initial values D0 and KdB0 at the start of control, theexpression of the tangent, that is, the formula for computation of KdB tis expressed by Expression 4:KdBt=−13.03×D/D0+KdB0+13.03  (Expression 4)

When Expression 5 obtained by modifying Expression 4 is substituted intoExpression 1, the formula for computation of target relative speedindicated as Expression 6 is obtained:KdBt/10=−1.303×D/D0+0.1×KdB0+1.303  (Expression 5)Vrt=−2.5×D ³×10^({−1.303×D/D0+0.1×KdB0−6.697})  (Expression 6)

A target relative speed is determined using Expression 6, anddeceleration is controlled based on this target relative speed and theactual relative speed. As a result, the vehicle is smoothly decelerated.However, expression 6 means that deceleration is controlled so that Vr=0when D=0. That is, in deceleration control using Expression 6,deceleration is controlled so that the relative speed becomes 0 when acollision occurs. For this reason, this deceleration control is notmatched with the feeling of the driver, and as a result, discomfort isbrought to the driver.

SUMMARY OF THE INVENTION

It is therefore an object of the invention is to provide a speed controlsystem for vehicles that makes it possible not to bring discomfort to adriver during deceleration.

According to one aspect, a speed control system is configured to:

detect a distance to a preceding vehicle;

detect a relative speed between a subject vehicle and the precedingvehicle;

compute a corrected distance condition evaluation index which isincreased with increase in the relative speed when approaching thepreceding vehicle and whose increase gradient becomes greater as thedistance to the preceding vehicle becomes shorter at each relative speedas an index indicating the condition of approaching or separating fromthe preceding vehicle by taking into consideration of a speed of thepreceding vehicle;set a threshold value of the corrected distance condition evaluationindex and takes an offset amount preset with respect to distance as zerovalue of distance at the threshold value of the corrected distancecondition evaluation index;check whether the corrected distance condition evaluation index ishigher than the threshold value of the corrected distance conditionevaluation index;

when the corrected distance condition evaluation index is higher thanthe threshold value of the corrected distance condition evaluationindex, set the threshold value of the corrected distance conditionevaluation index as a target value of the corrected distance conditionevaluation index;

compute the target deceleration of the subject vehicle based on a targetrelative speed corresponding to the target value set by the target valuesetting means and an actual relative speed; and

start deceleration control to decelerate the subject vehicle so thatdeceleration of the subject vehicle becomes equal to the targetdeceleration.

According to a second aspect, a speed control system is configured to:

detect a distance to an object ahead;

detect a relative speed between a subject vehicle and the object ahead;

store a correction formula for computation of a corrected targetrelative speed obtained by correcting a formula for computation of atarget relative speed with an offset amount indicating the targetrelative speed when the distance is zero,

wherein the formula for computation of target relative speed isdetermined from a distance condition evaluation index relationalexpression and a tangential expression so that a target relative speedis determined based on the distance,

wherein, the distance condition evaluation index relational expressionindicates a relation among the distance condition evaluation indexindicating condition of distance to the object ahead, a distance to theobject ahead and the relative speed, so that the distance conditionevaluation index is increased with increase in the relative speed whenapproaching the object ahead and an increase gradient of the distancecondition evaluation index becomes greater as the distance to the objectahead becomes shorter at identical relative speed, and

wherein the tangential expression indicates a tangent of a curverepresented by the distance condition evaluation index relationalexpression, and is determined by differentiating the distance conditionevaluation index relational expression by the distance, therebyindicating a relation between the distance condition evaluation indexand the distance;

compute a target relative speed from the correction formula forcomputation of target relative speed and a distance actually detected bythe distance detecting means;

compute a target deceleration from a target relative speed computed bythe target relative speed computing means and a relative speed actuallydetected by the relative speed detecting means; and

perform deceleration control on the subject vehicle based on the targetdeceleration.

According to a third aspect, a speed control system is configured to:

detect a distance to an object ahead;

detect a relative speed between a subject vehicle and the object ahead;

store a correction formula for computation of a corrected targetrelative speed obtained by correcting a formula for computation of atarget relative speed with an offset amount indicating the distance whenthe relative speed is zero,

wherein the formula for computation of target relative speed isdetermined from a distance condition evaluation index relationalexpression and a tangential expression so that a target relative speedis determined based on the distance,

wherein, the distance condition evaluation index relational expressionindicates a relation among the distance condition evaluation indexindicating condition of distance to the object ahead, a distance to theobject ahead and the relative speed, so that the distance conditionevaluation index is increased with increase in the relative speed whenapproaching the object ahead and an increase gradient of the distancecondition evaluation index becomes greater as the distance to the objectahead becomes shorter at identical relative speed, and

wherein the tangential expression indicates a tangent of a curverepresented by the distance condition evaluation index relationalexpression, and is determined by differentiating the distance conditionevaluation index relational expression by the distance, therebyindicating a relation between the distance condition evaluation indexand the distance;

compute a target relative speed from the correction formula forcomputation of target relative speed and a distance actually detected bythe distance detecting means;

compute a target deceleration from a target relative speed computed bythe target relative speed computing means and a relative speed actuallydetected by the relative speed detecting means; and

perform deceleration control on the subject vehicle based on the targetdeceleration.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a block diagram illustrating a drive assistance system in afirst embodiment;

FIG. 2 is an explanatory diagram illustrating varying brake discriminantKdBc and current value (KdBct) of corrected distance conditionevaluation index KdBc depending on inter-vehicle distance Doffset;

FIG. 3 is a flowchart illustrating acceleration/deceleration controlprocessing performed in the first embodiment;

FIG. 4 is an explanatory diagram illustrating a threshold value ofcorrected distance condition evaluation index KdBc used when the actualinter-vehicle distance D to the preceding vehicle becomes shorter thanoffset distance Doffset, in a modification;

FIG. 5 is an explanatory diagram illustrating comparison between curveC1 represented by a formula for computation of target relative speed andcurve C2 represented by a formula for computation of corrected targetrelative speed;

FIG. 6 is a flowchart illustrating control performed in a secondembodiment;

FIG. 7 is a flowchart illustrating details of processing of FIG. 6;

FIG. 8 is an explanatory diagram illustrating comparison between curveC3 represented by a formula for computation of corrected target relativespeed and curve C1 for computation of target relative speed;

FIG. 9 is a block diagram illustrating a drive assistance system in athird embodiment;

FIG. 10A is a view illustrating examples of a traffic separation linem1, a reflector plate m5, a guard rail m3, and a curb m4;

FIG. 10B is a view illustrating examples of a pole m2 and a delineatorm6;

FIG. 11 is a flowchart illustrating assist control for braking forceperformed in the third embodiment;

FIG. 12 is an explanatory diagram illustrating comparison between curveC4 represented by a formula for computation of corrected target relativespeed in the third embodiment and curve C1 represented by a formula forcomputation of target relative speed;

FIG. 13 is an explanatory diagram illustrating comparison between curveC5 represented by a formula for computation of corrected target relativespeed in a fourth embodiment and curve C1 represented by a formula forcomputation of target relative speed; and

FIG. 14 is an explanatory diagram illustrating brake discriminant KdBcin a related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

Referring to FIG. 1, a speed control system for vehicles is applied to adrive assistance system as an example. The drive assistance system ismounted on a subject vehicle (not shown) and constructed of a vehiclestability control (VSC) electronic control unit (ECU) 10, a steeringangle sensor 20, a G sensor 30, a yaw rate sensor 40, engine (ENG) ECU50, a radar 70, an operation switch group (SW) 80, and a vehicle controlECU 100.

The VSC ECU 10 controls a brake actuator (not shown) that appliesbraking force to the subject vehicle and has a vehicle stability controlfunction for suppressing skidding of the subject vehicle. The VSC ECU 10receives information on a target deceleration through an in-vehicle LANand controls the brake actuator so that this target deceleration isproduced in the subject vehicle. The VSC ECU 10 transmits information onthe speed (subject vehicle speed) Vs0 and braking pressure of thesubject vehicle to the in-vehicle LAN.

The steering angle sensor 20 detects information on the steering angleof the steering gear of the subject vehicle and transmits the detectedinformation on steering angle to the in-vehicle LAN.

The G sensor 30 is an acceleration sensor that detects accelerationproduced in the direction of the length of the subject vehicle(longitudinal G) and acceleration produced in the lateral (right andleft) direction (lateral G). The G sensor 30 transmits information onthe detected longitudinal G and lateral G to the in-vehicle LAN.

The yaw rate sensor 40 detects the angular velocity (yaw rate) aroundthe vertical axis of the subject vehicle and transmits information onthe detected yaw rate to the in-vehicle LAN.

The ENG ECU 50 receives information on target acceleration from thein-vehicle LAN and controls a throttle actuator (not shown), so that thesubject vehicle produces the target acceleration.

The radar 70 irradiates a predetermined range in front of the subjectvehicle with, for example, laser light and receives the resultingreflected light to detect the following: the inter-vehicle distance Dfrom the subject vehicle to the preceding vehicle, the relative speed Vrbetween the subject vehicle and the preceding vehicle; the displacementbetween the central axis of the subject vehicle in the direction ofwidth and the central axis of the preceding vehicle (lateraldisplacement), and the like. The radar 70 outputs the detectedinformation to the vehicle control ECU 100.

The operation SW 80 is a group of switches operable by the driver of thesubject vehicle and information on the operation of the switch group isoutputted to the vehicle control ECU 100. The vehicle control ECU 100 isconstructed based on a microcomputer and is constructed of CPU, ROM,RAM, I/O, and a bus connecting them which are all known.

The vehicle control ECU 100 uses the brake discriminant KdBc illustratedin FIG. 2 to determine deceleration control start timing when thesubject vehicle travels following the preceding vehicle. The vehiclecontrol ECU 100 carries out deceleration control to produce thefollowing in the subject vehicle: a target relative speed Vrctcorresponding to the brake discriminant KdBc and a targetacceleration/deceleration G_(Dp) computed from the relative speed Vrcpof the subject vehicle. The vehicle control ECU 100 thereby obtains thefeeling of deceleration comfortable for the driver in various drivingscenes. The brake discriminant KdBc illustrated in FIG. 14 is describedin U.S. patent application Ser. No. 12/151,998. Therefore, thedescription of the brake discriminant KdBc will be omitted, anddescription will be given with a focus on a difference.

Unlike the brake discriminant KdBc illustrated in FIG. 14, the brakediscriminant KdBc used in the first embodiment is so constructed thatthe zero value (=0 [m]) of the inter-vehicle distance D is taken as theclosest inter-vehicle distance Dq. More specifically, as illustrated inFIG. 2, an offset distance Doffset is provided so that decelerationcontrol is completed at the closest inter-vehicle distance Dq. The brakediscriminant KdBc in FIG. 2 is given by Expression 8:KdBc=−23.76×log₁₀ Doffset+76.96  (Expression 8)

The brake discriminant KdBc of Expression 8 is set with an inter-vehicledistance equal to or longer than the closest inter-vehicle distance Dq.In the drive assistance system, therefore, it is possible to carry outdeceleration control so that deceleration is completed at aninter-vehicle distance equal to or longer than the closest inter-vehicledistance Dq. As a result, the target deceleration of the subject vehicledoes not have an inflection point at some midpoint in the process of theinter-vehicle distance being shortened and discomfort will not bebrought to the driver during deceleration.

The deceleration control processing is performed by the vehicle controlECU 100 by executing the processing shown in FIG. 3. First, at S10 inFIG. 3, input signals are acquired from various sensors. At S20, thecurrent value KdBcp of corrected distance condition evaluation indexKdBc is computed. At S30, a threshold value KdBct on the brakediscriminant KdBc corresponding to the inter-vehicle distance Doffsetobtained by adding the offset distance to the actual inter-vehicledistance to the preceding vehicle is computed from Expression 8.

At S40, it is checked whether the current value KdBcp of correcteddistance condition evaluation index KdB is higher than the thresholdvalue KdBct on the brake discriminant KdBc. When an affirmativedetermination (Yes) is made here, the processing proceeds to S50. When anegative determination (No) is made, the processing returns to S10 andthe above processing is repeated.

At S50, as illustrated in FIG. 2, a target acceleration/decelerationG_(Dp) to be produced in the subject vehicle is computed. This targetacceleration/deceleration G_(Dp) is given by Expression 9:G _(Dp)=(Vrcp−Vrct)/T  (Expression 9)

Vrcp in Expression 9 represents the actual relative speed of the subjectvehicle. Vrct is a target relative speed obtained by taking thefollowing procedure: a threshold value KdBct on the brake discriminantKdBc corresponding to the inter-vehicle distance Doffset, obtained byadding the offset distance to the actual inter-vehicle distance to thepreceding vehicle, is determined from the brake discriminant KdBc. Thisthreshold value KdBct is substituted into Expression 10 below. ThisExpression 10 is described in U.S. patent application Ser. No.12/151,998, and thus the description of the expression will be omittedhere. Here, α is a coefficient not more than 1, and it has been foundthat α=0.3 or so is most appropriate. T is a divisor for transformingthe difference between the present relative speed Vrcp of the subjectvehicle and the target relative speed Vrct to theacceleration/deceleration G_(Dp) of the subject vehicle taken as atarget.

$\begin{matrix}{{KdBc} = {10 \times \log\left\{ {{{- 2} \times \frac{{{Vr}} + {\alpha \times {{Vb}}}}{D^{3} \times 5 \times 10^{- 8}}}} \right\}}} & \left( {{Expression}\mspace{14mu} 10} \right)\end{matrix}$

At S60, it is checked whether a termination condition for decelerationcontrol has been met. For example, the following events can be used forthis deceleration control termination condition: the subject vehicle isstopped; and the current value KdBcp of corrected distance conditionevaluation index KdBc falls below the threshold value KdBct on the brakediscriminant KdBc. When the control termination condition has not beenmet, the processing of S10 and the following steps is repeated.

In the above drive assistance system, the brake discriminant KdBc is setwith the inter-vehicle distance equal to or longer than the presetoffset distance Doffset. Therefore, deceleration control can beperformed so that deceleration is completed at the inter-vehicledistance equal to this offset distance Doffset. As a result, the targetdeceleration of the subject vehicle does not have an inflection point atsome midpoint in the process of the inter-vehicle distance beingshortened, and the driver will not feel discomfort driver duringdeceleration.

Second Embodiment

In the second embodiment as well, a drive assistance system may beconfigured in the same way as the first embodiment.

However, the formula for computation of corrected target relative speed,expressed by Expression 11, is stored in a storage device, such as theROM, internal to the vehicle control ECU 100 or a storage deviceprovided separately from the vehicle control ECU 100:

$\begin{matrix}{{Vrtafter} = {{{Vr}\; 0} - {\frac{{{Vr}\; 0} - {Vroffset}}{{Vr}\; 0} \times \left\{ {{{Vr}\; 0} + {2.5 \times D^{3} \times 10^{({{\frac{- 1.303}{D\; 0} \times D} + {0.1 \times {KdB}\; 0} - 6.697})}}} \right\}}}} & \left( {{Expression}\mspace{14mu} 11} \right)\end{matrix}$

When a deceleration control start condition is met, the vehicle controlECU 100 carries out deceleration control using Expression 11 above.

Expression 11 is obtained by taking the following measure. With respectto the formula for computation of target relative speed expressed byExpression 6, the target relative speed at distance D=0 is offset to thepositive side by the offset relative speed Vroffset. From D=D0 to 0, theformula for computation is varied at a relative speed obtained bymultiplying the rate A of change in relative speed indicated inExpression 12 by the target relative speed Vrt that can be computed byExpression 6. The formula for computation is obtained by transposing theVr0 term in Expression 13 to the right side after the modification ofexpression represented by Expression 13.

$\begin{matrix}{a = {{rate} = \frac{{{Vr}\; 0} - {Vroffset}}{{Vr}\; 0}}} & \left( {{Expression}\mspace{14mu} 12} \right) \\{{{{Vr}\; 0} - {Vrtafter}} = {{a \times \left( {{{Vr}\; 0} - {Vrt}} \right)} = {\frac{{{Vr}\; 0} - {Vroffset}}{{Vr}\; 0} \times \left\{ {{{Vr}\; 0} + {2.5 \times D^{3} \times 10^{({{\frac{- 1.303}{D\; 0} \times D} + {0.1 \times {KdB}\; 0} - 6.697})}}} \right\}}}} & \left( {{Expression}\mspace{14mu} 13} \right)\end{matrix}$

FIG. 5 is a diagram illustrating comparison between a curve C1represented by Expression 6 (formula for computation of target relativespeed) and a curve C2 represented by Expression 11 (formula forcomputation of corrected target relative speed). In the curve C2represented by the formula for computation of corrected target relativespeed, as indicated by FIG. 5 as well, the distance Dvr0 at which therelative speed is 0 is greater than zero.

The vehicle control ECU 100 in the second embodiment executes theprocessing illustrated in FIG. 6 in a predetermined cycle.

At S100, first, input signals are acquired from various sensors. AtS110, subsequently, it is checked whether a deceleration control startcondition has been met. In the second embodiment as well as the firstembodiment, the following is taken as this deceleration control startcondition. The current value KdBcp of corrected distance conditionevaluation index KdB is higher than the threshold value KdBct on thebrake discriminant KdBc. When an affirmative determination is made here,the processing proceeds to S120. When a negative determination is made,the processing returns to S100 and the above processing is repeated.

At S120, the initial value KdB0 of distance condition evaluation indexKdB is computed. Specifically, the commutation is performed bysubstituting the following into Expression 14: the distance D0 to thepreceding vehicle detected by the radar 70 and acquired at theimmediately preceding step, or S100 and the relative speed Vr0 which isa rate of time change in this distance D0. Expression 14 is obtained bymodifying Expression 1:

$\begin{matrix}{{KdB} = {10 \times {\log\left( {{{- 2} \times \frac{Vr}{D^{3}} \times \frac{1}{5 \times 10^{- 8}}}} \right)}}} & \left( {{Expression}\mspace{14mu} 14} \right)\end{matrix}$

At S130, subsequently, deceleration control using Expression 11 isperformed. The processing of S130 will be described in detail later. AtS140, subsequently, it is checked whether a deceleration controltermination condition has been met. For example, the following eventscan be used for this deceleration control termination condition: thesubject vehicle is stopped; and the current value KdBcp of correcteddistance condition evaluation index KdBc falls below the threshold valueKdBct on the brake discriminant KdBc. When the deceleration controltermination condition has not been met, the processing of S100 and thefollowing steps are repeated.

Details of the processing of S130 is shown in FIG. 7. At S131, thecorrected target relative speed Vrtafter is computed by substituting thefollowing into Expression 11: the current value Dp of the distance tothe preceding vehicle acquired at the immediately preceding step, orS100; the initial value KdB0 of distance condition evaluation indexcomputed at S120; and the distance D0 at the start of control and therelative speed Vr0 used to compute this initial value KdB0.

At S132, subsequently, the current value Dp of the distance to thepreceding vehicle is differentiated to determine the present relativespeed Vrp between the subject vehicle and the preceding vehicle. Thispresent relative speed Vrp and the corrected target relative speedVrtafter computed at S131 are substituted into Expression 15. A targetrelative deceleration G_(Dp) to be produced in the subject vehicle isthereby computed. In Expression 15, T is a divisor for transforming thedifference between the present relative speed Vrp of the subject vehicleand the corrected target relative speed Vrtafter into the targetrelative deceleration G_(Dp) and is appropriately set.G _(Dp)=(Vrp−Vrtafter)/T  (Expression 15)

At S133, the target relative deceleration G_(Dp) computed at S132 isoutputted to the VSC ECU 10. The VSC ECU 10 carries out decelerationcontrol using a brake actuator, not shown, so that the target relativedeceleration G_(Dp) inputted from the vehicle control ECU 100 isproduced in the subject vehicle.

In the second embodiment, as described above, deceleration control isperformed using a formula for computation of corrected target relativespeed (Expression 11) obtained by offsetting the target relative speedat distance D=0 to the positive side by the relative speed Vroffset. Thecorrected target relative speed Vrtafter obtained by Expression 11becomes zero before the distance D to the preceding vehicle becomeszero. For this reason, it is possible to match deceleration control withthe feeling of the driver more and achieve smooth deceleration control.

Modification to Second Embodiment

The formula for computation of corrected target relative speed(Expression 11) used in the second embodiment is an expression obtainedby multiplying the target relative speed Vrt that can be computed byExpression 6 by the ratio “a”. Instead, Expression 16, obtained byadding the following term to the target relative speed Vrt that can becomputed by Expression 6, may be used in place of Expression 11: a termobtained by multiplying the ratio of the travel distance (D0-D) sincethe start of control to the distance D0 at the start of control by theoffset relative speed Vroffset:

$\begin{matrix}{{Vrtafter} = {{{- 2.5} \times D^{3} \times 10^{({{\frac{- 1.303}{D\; 0} \times D} + {0.1 \times {KdB}\; 0} - 6.697})}} + {\left( \frac{Vroffset}{Do} \right) \times \left( {{D\; 0} - D} \right)}}} & \left( {{Expression}\mspace{14mu} 17} \right)\end{matrix}$

FIG. 8 illustrates comparison between a curve C3 represented by theformula for computation of corrected target relative speed of Expression16 and the curve C1 represented by Expression 6 (formula for computationof target relative speed).

Third Embodiment

In the third embodiment, a drive assistance system is constructed of, asshown in FIG. 9, a radar 310, a vehicle speed sensor 320, a brake switch(SW) 330, a braking pressure sensor 340, an operation switch (SW) 350, abrake ECU 360, a brake actuator 370, a CCD camera 380 a, an imagerecognition processor 380 b, and an automobile navigation system 390.

The radar 310 irradiates a predetermined range in front of the subjectvehicle with, for example, laser light and receives the resultingreflected light. The radar thereby detects, for example, the followingitems illustrated in FIGS. 10A and 10B: distance to a forward object,such as a reflector plate m5 that is installed at a boundary of a curvedroad or in proximity thereto and delivers a reflected light intensityequal to or higher than a predetermined intensity or a road-affixedobject such as a delineator m6 belonging to a road; lateral displacementbetween the central axis of the subject vehicle in the direction ofwidth and the central axis of the road-affixed object; and the like. Theradar 310 outputs the detected information to the brake ECU 360.

The vehicle speed sensor 320 detects travel speed of the subjectvehicle. A road-affixed object detected by the above radar 310 isinstalled and fixed on the road. Therefore, the relative speed Vrbetween a road-affixed object and the subject vehicle is equal to thespeed of the subject vehicle. The sign of the relative speed Vr isdefined as follows: when the subject vehicle approaches a road-affixedobject, the sign is negative (−); and when the subject vehicle separates(moves away) from a road-affixed object, the sign is positive (+).

The brake SW 330 detects brake operation performed by the driver of thesubject vehicle. When the brake pedal is pressed down or operated, thebrake SW outputs an ON signal. When the pedal is not operated, the brakeSW outputs an OFF signal.

The braking pressure sensor 340 detects brake fluid pressure produced ina brake device (not shown), when the brake pedal is operated by thedriver of the subject vehicle. The brake device presses, for example, adisc pad against a disc rotor fixed on a wheel with a strengthcorresponding to this brake fluid pressure to produce braking force andthereby decelerates the subject vehicle. Therefore, the decelerationproduced in the subject vehicle by the driver of the subject vehicleoperating the brake pedal can be estimated from the brake fluid pressureas the result of this brake operation.

The operation SW 350 is operated by the driver of the subject vehicleand a resulting operation signal is inputted to the brake ECU 360. Whenthe brake ECU 360 assists and controls brake operation by the driver ofthe subject vehicle, the operation SW 350 supplies an instruction toadjust the degree of deceleration to the brake ECU 360 to gently orstrongly decelerate the subject vehicle.

The brake actuator 370 adjusts the brake fluid pressure of the brakedevice to an arbitrary pressure according to an instruction signal fromthe brake ECU 360 described later.

The CCD camera 380 a is an imaging means for picking up an image of anarea within a predetermined range in front of the subject vehicle, andoutputs a picked-up image to the image recognition processor 380 b. Theimage recognition processor 380 b carries out predetermined imagerecognition processing on a picked-up image outputted from the CCDcamera 380 a. The image recognition processor thereby recognizes aroad-affixed object. Example of such objects include: traffic (travellane) separation line m1, pole m2, guardrail m3, curbstone m4, and thelike provided at a boundary of a curved road in front of the subjectvehicle or in proximity thereto illustrated in FIGS. 10A and 10B.

The image recognition processor 380 b determines the relative positionbetween the road-affixed object and the subject vehicle. The processor380 b outputs information on the type of the road-affixed object and therelative position thereof to the brake ECU 360.

The automobile navigation system 390 is constructed of: a positiondetection unit including a geomagnetic sensor, a gyroscope, a distancesensor, a GPS receiver for global positioning system (GPS) for detectingthe position of the subject vehicle based on radio waves from satellitesin the known manner; a road map data storage unit for storing road mapdata; a color display using liquid crystal, CRT, or the like; and acontrol circuit.

The road map data includes link data and node data for representing aroad in a map by a link and nodes. The link data and node data includesinformation on the coordinates of the start point, end point, and linklength of each link, a width between traffic separation lines, and thecurvature radius of a road. When the automobile navigation system 390receives an instruction from the brake ECU 360, the navigation system390 identifies the present position of the subject vehicle and outputsthe following data: link data and node data related to any curved roadpresent within a predetermined distance in front of the subject vehicle.

The brake ECU 360 assists and controls the braking force of the brakedevice according to input signals from the above various sensors andswitches. The brake ECU 360 carries out the assist and control so thatthe following is implemented when the subject vehicle enters a curvedroad ahead in the traveling direction of the subject vehicle or istraveling along a curved road. When the subject vehicle approaches aroad-affixed object and the driver performs braking operation, collisionwith the road-affixed object is avoided and the favorable feeling ofdeceleration is obtained.

In the third embodiment as well, the formula for computation ofcorrected target relative speed, expressed by Expression 11, is storedin a predetermined storage device, such as ROM, internal to the brakeECU 360 or a storage device provided separately from the brake ECU 360.The brake ECU 360 carries out assist and control using this formula forcomputation of corrected target relative speed. However, while theoffset relative speed Vroffset takes a positive value in the secondembodiment, the offset relative speed Vroffset is set to a negativevalue in the third embodiment.

Assist control for braking force (braking assist control) performed bythe brake ECU 360 is executed as shown in FIG. 11. This assist controlis performed when the subject vehicle enters a curved road ahead in thetraveling direction of the subject vehicle or is traveling along acurved road. It is checked based on an output signal from the imagerecognition processor 380 b or the automobile navigation system 390whether the subject vehicle enters a curved road or is traveling along acurved road.

At S200, first, the brake ECU 360 acquires input signals from varioussensors and switches 310 to 350. At S210, it is checked whether adetection signal from the brake SW 30 has changed from OFF to ON. Thatis, at S210, it is checked whether the driver of the subject vehicle hasstarted brake operation.

When it is determined at S210 that the detection signal from the brakeSW 330 has changed to ON, the processing proceeds to S220, and theinitial evaluation index (initial value of distance condition evaluationindex) KdB0 is computed. Specifically, the initial index KdB0 iscomputed as follows. Using the input signals acquired at the immediatelypreceding step, or S200, the distance D0 to a road-affixed objectdetected by the radar 310 is determined. Further, the vehicle speed Vs0of the subject vehicle detected by the vehicle speed sensor 320 isdetermined as the relative speed Vr0. D0 and Vr0 are substituted intoExpression 14 described in relation to the second embodiment.

At S230, subsequently, a corrected target relative speed Vrtafter iscomputed by substituting the following into Expression 11: the initialvalue KdB0 of distance condition evaluation index computed at S220; andthe distance D to the road-affixed object acquired at the immediatelypreceding step, or S200.

FIG. 12 illustrates comparison between a curve C4 represented by theformula for computation of corrected target relative speed (Expression11) in the third embodiment and a curve C1 illustrated also in FIG. 5.This curve C1 is a curve represented by a formula for computation oftarget relative speed (Expression 6) without the offset relative speedVroffset provided.

In the curve C1, as illustrated in FIG. 12, the relative speed Vrbecomes zero when distance D=0. Therefore, the following takes placewhen a road-affixed object is taken as the object of relative speed Vrand the curve C1 is used for deceleration control in curve traveling.When the distance D to the road-affixed object becomes zero, therelative speed Vr of the subject vehicle relative to the road-affixedobject becomes zero, that is, the subject vehicle speed Vs0 becomeszero. In the curve C4, meanwhile, the relative speed when the distancebecomes zero is equal to the offset relative speed Vroffset (<0). In thethird embodiment, therefore, deceleration control is performed so thatthe following is implemented: the relative speed Vr when the distance Dbecomes zero (that is, the subject vehicle speed Vs0 in the thirdembodiment) becomes equal to the offset relative speed Vroffset (<0).

The value of the above offset relative speed Vroffset may be a presetconstant value or a value determined based on a curvature acquired oneach curved road. Further, the offset relative speed Vroffset may bedetermined by taking the coefficient μ of friction on a road intoaccount in addition to the curvature. The curvature of a curved road canbe determined based on an output signal from the image recognitionprocessor 380 b or the automobile navigation system 390. The coefficientμ of friction on a road can be determined by providing a road conditiondetector and acquiring an output signal from the road conditiondetector. Alternatively, the following measure may be taken: roadconditions, such as dry, wet, snow covering, icy, and the like arebrought beforehand into correlation with coefficients μ of friction on aroad; the driver is made to select dry, wet, snow covering, icy, or thelike; and the coefficient μ of friction on the road is determined fromthe selected road condition. The curvature of a curved road or acoefficient μ of friction on a road may be acquired by wirelesscommunication with a source external to the subject vehicle, such asinter-vehicle communication and vehicle roadside communication.

At S240, the present relative speed Vrp (that is, subject vehicle speedVs0) between the subject vehicle and the road-affixed object based on asignal from the vehicle speed sensor 320. This present relative speedVrp and the corrected target relative speed Vrtafter computed at S230are substituted into Expression 15 as in the second embodiment. A targetrelative deceleration G_(Dp) to be produced in the subject vehicle isthereby computed.

At S250, it is checked whether a time to collision TTC, which indicatesa time it takes for the subject vehicle to collide with a road-affixedobject, is shorter than a predetermined time Tref. When it is determinedas the result of the determination processing of S260 that TTC<Tref, theprocessing proceeds to S260. When it is determined that TTC≧Tref, theprocessing proceeds to S290.

When it is determined that TTC≧Tref, there is a sufficient time tocollision TTC when brake operation is started by the driver. It isexpected that collision with a road-affixed object can be sufficientlyavoided by brake operation or the like by the driver him/herself. AtS290, therefore, assist control for braking force by this brake controlunit is not performed.

At S260, the deceleration dVr/dt produced in the subject vehicle isestimated based on the breaking pressure produced as the result of brakeoperation by the driver. At S270, it is checked whether the estimateddeceleration dVr/dt corresponding to the brake operation by the driveris greater than the target relative deceleration G_(Dp) computed atS240. A deceleration is represented as a negative (−) value. Therefore,that “the estimated deceleration dVr/dt corresponding to the brakeoperation by the driver is greater than the target relative decelerationG_(Dp)” means the following: the brake operation by the driver producestoo low degree of deceleration to decelerate the subject vehicle withthe target relative deceleration G_(Dp).

When an affirmative determination is made at S270, therefore, theprocessing proceeds to S280 and braking force assist control isperformed at S280. That is, this braking force assist control isperformed in cases where: the time to collision TTC it takes for thesubject vehicle to collide with a road-affixed object is shorter than apredetermined time Tref; and the subject vehicle cannot be deceleratedwith the target relative deceleration G_(Dp) by brake operation by thedriver of the subject vehicle.

In the braking force assist control performed at S280, the followingprocessing is performed: a braking pressure for producing the targetrelative deceleration G_(Dp) computed at S240 is determined from aprepared map and the brake actuator 370 is controlled so as to producethis braking pressure; or the actual deceleration of the subject vehicleis detected and the braking pressure is adjusted by the brake actuator370 so that this actual deceleration becomes equal to the targetrelative deceleration G_(Dp).

When it is conversely determined at S270 that the estimated decelerationdVr/dt corresponding to brake operation by the driver is lower than thetarget relative deceleration G_(Dp), the subject vehicle can bedecelerated with a deceleration higher than the target relativedeceleration G_(Dp) by brake operation by the driver. Since it isexpected that sufficient deceleration is produced by brake operation bythe driver, it is unnecessary to carry out assist control by the brakecontrol unit. Therefore, the processing proceeds to S290 and brakingforce assist control is not performed.

At S300, it is checked whether a termination condition for the assistcontrol has been met. For example, the following events can be used forthis control termination condition: the subject vehicle is stopped; andthe time to collision TTC exceeds the predetermined time Tref. When thecontrol termination condition has not been met, the processing of S200and the following steps is repeated.

In the third embodiment, as described above, deceleration control isperformed using the formula for computation of corrected target relativespeed (Expression 11). This formula is obtained by offsetting the targetrelative speed Vr (equal to the subject vehicle speed Vs0 in the thirdembodiment) at the distance D to a road-affixed object=0 to the negativeside by the relative speed Vroffset. Therefore, it is possible to bringthe relative speed Vr (that is, the subject vehicle speed Vs0) when thedistance D to the road-affixed object becomes zero to the negative sideand achieve smooth deceleration control during curve traveling. Theabove negative side is the side on which the vehicle does not approachthe road-affixed object.

Modification to Third Embodiment

In the third embodiment, the brake ECU 360 carries out assist control.Because of assist control, that brake operation is started by the driverof the subject vehicle is taken as a criterion for starting assistcontrol. Instead, deceleration control may be performed regardless ofbrake operation by the driver.

In this case, the following measure is taken: a deceleration targetKdBssdc representing an index of the timing with which the braking forceof the brake device is controlled is computed from the normaldeceleration of the subject vehicle, the distance to a road-affixedobject, and the actual relative speed; and the time when it isdetermined that the current value KdBp of distance condition evaluationindex exceeds the deceleration target KdBssdc is taken as the time tostart deceleration control.

The above deceleration target KdBssdc is computed by Expression 17, andthe current value KdBp of distance condition evaluation index iscomputed by Expression 14:

$\begin{matrix}{{KdBssdc} = {10 \times \left( {8 + {\log{\frac{nd}{7.5 \times D^{2} \times {Vr}}}}} \right)}} & \left( {{Expression}\mspace{14mu} 17} \right)\end{matrix}$

Expression 17 is derived in the following manner. First, Expression 1 isdifferentiated to obtain Expression 18 below:

$\begin{matrix}{{\frac{\mathbb{d}{Vr}}{\mathbb{d}D} \times \frac{\mathbb{d}D}{\mathbb{d}t}} = {7.5 \times D^{2} \times 10^{\{{{({{{KdB}}/10})} - 8}\}} \times {Vr}}} & \left( {{Expression}\mspace{14mu} 18} \right)\end{matrix}$

Since Expression 18 represents deceleration, the normal deceleration ndof the subject vehicle and the deceleration target KdBssdc at that timecan be expressed as in Expression 19. The normal deceleration of thesubject vehicle is, for example, normal deceleration produced in thesubject vehicle by the driver performing driving operation to deceleratethe subject vehicle.nd=7.5×D ²×10^({(|KdB ssdc|/10)−8}) ×Vr  (Expression 19)

Expression 20 is obtained by modifying Expression 19.

$\begin{matrix}{10^{\{{{({{{{KdB}\mspace{14mu}{ssdc}}}/10})} - 8}\}} = \frac{nd}{7.5 \times D^{2} \times {Vr}}} & \left( {{Expression}\mspace{14mu} 20} \right)\end{matrix}$

Expression 17 represents Expression 20 by logarithm. As described above,the normal deceleration nd in Expression 17 is normal decelerationproduced in the subject vehicle by the driver performing drivingoperation to decelerate the subject vehicle. Instead, the normaldeceleration may be deceleration produced in the subject vehicle byengine brake.

When Vroffset in Expression 16 is set to a negative value, Expression 16can be used also in the third embodiment.

Fourth Embodiment

In the fourth embodiment as well, a drive assistance system is the sameas that of the second embodiment. In the second embodiment, decelerationcontrol is performed using a formula for computation of corrected targetrelative speed (Expression 11) having an offset relative speed Vroffset(>0). In the fourth embodiment, however, the offset relative speedVroffset is not provided, and instead, an offset distance Doffset havinga positive value is provided and deceleration control is performed usingExpression 21 below. For this reason, the formula for computation ofcorrected target relative speed of Expression 21 is stored in apredetermined storage device:

$\begin{matrix}{{Vrtafter} = {{- 2.5} \times {Dafter}^{3} \times 10^{({{\frac{- 1.303}{D\; 0} \times {Dafter}} + {0.1 \times {KdB}\; 0} - 6.697})}}} & \left( {{Expression}\mspace{14mu} 21} \right)\end{matrix}$

In Expression 21, Dafter is a corrected target minimum inter-vehicledistance, which is expressed by Expression 22.

$\begin{matrix}{{Dafter} = {{D\; 0} - \frac{D\; 0 \times \left( {{D\; 0} - D} \right)}{{D\; 0} - {Doffset}}}} & \left( {{Expression}\mspace{14mu} 22} \right)\end{matrix}$

Expression 21 is obtained by substituting a corrected target minimuminter-vehicle distance Dafter for D in Expression 6. Expression 22 canbe computed from a ratio “b” indicated in FIG. 13. This ratio “b” is aratio of a distance determined by the formula for computation of targetrelative speed of Expression 6 at a given relative speed Vr to adistance determined by the formula for computation of corrected targetrelative speed of Expression 21. This ratio “b” can be expressed as inthe right-hand side of Expression 23 as a general formula. When therelative speed Vr is zero, the ratio “b” can be expressed as in themiddle part of Expression 23. Expression 22 can be obtained by modifyingExpression 23:

$\begin{matrix}{b = {\frac{{D\; 0} - {Doffset}}{D\; 0} = \frac{{D\; 0} - D}{{D\; 0} - {Dafter}}}} & \left( {{Expression}\mspace{14mu} 23} \right)\end{matrix}$

The control performed by the vehicle control ECU 100 in the fourthembodiment is the same as in the second embodiment except that anexpression used in deceleration control is different. The followingevent may be used for the deceleration control start condition inaddition to the condition in the second embodiment or in place of thecondition in the second embodiment: an instruction to carry out controlfor following the preceding vehicle is given by the driver. This controlis speed control to vary the speed of the subject vehicle according tothe speed of the preceding vehicle.

In deceleration control using Expression 21 (curve C5), as seen fromFIG. 13 as well, deceleration control is performed so that with theoffset distance Doffset, the distance when the relative speed Vr becomeszero has a positive value. That is, deceleration control is performedwith the closest inter-vehicle distance ensured. Therefore, the fourthembodiment is suitable for control for following the preceding vehicle,such as adaptive cruise control.

A favorable value of the above offset distance Doffset may be determinedas follows. First, Expression 24 can be obtained by modifying Expression10:

$\begin{matrix}{{Vr} = {{{- 2.5} \times D^{3} \times 10^{({\frac{{KdB}\mspace{14mu} c}{10} - 8})}} + {\alpha \times {Vb}}}} & \left( {{Expression}\mspace{14mu} 24} \right)\end{matrix}$

As described in U.S. patent application Ser. No. 12/151,998, the pointat which the driver starts brake operation is distributed on one curvein a graph. In this graph, the distance D is taken as horizontal axisand the corrected distance condition evaluation index KdBc is taken asvertical axis. This curve can be represented by Expression 25 below.Expression 25 indicates the relation between the corrected distancecondition evaluation index KdBc and the distance D when the driverstarts brake operation. Therefore, Expression 25 can be used todiscriminate a point at which brake operation is started. For thisreason, this expression may also be designated as brake discriminant.KdBc−β log₁₀ D−γ=0  (Expression 25)where, β and γ are constants determined based on experiment.

When Expression 25 is substituted into Expression 24, Expression 26below can be obtained:

$\begin{matrix}{{Vr} = {{{- 2.5} \times D^{3} \times 10^{({\frac{{\beta \times \log_{10}D} + \gamma}{10} - 8})}} + {\alpha \times {Vb}}}} & \left( {{Expression}\mspace{14mu} 26} \right)\end{matrix}$

When Vr=0 in Expression 26, Expression 27 is obtained. Expression 28 isobtained by modifying Expression 27:

$\begin{matrix}{{{2.5 \times D^{3} \times 10^{({\frac{{\beta \times \log_{10}D} + \gamma}{10} - 8})}} - {\alpha \times {Vb}}} = 0} & \left( {{Expression}\mspace{14mu} 27} \right) \\{10^{({\frac{{\beta \times \log_{10}D} + \gamma}{10} - 8})} = {\frac{\alpha \times {Vb}}{2.5 \times D^{3}} = \frac{{\alpha/2.5} \times \;{Vb}}{D^{3}}}} & \left( {{Expression}\mspace{14mu} 28} \right)\end{matrix}$

When the logarithms of both sides of Expression 28 are taken, Expression29 is obtained. When Expression 30 is sequentially modified, Expressions30, 31, and 32 are obtained:

$\begin{matrix}{{\frac{{\beta \times \log_{10}D} + \gamma}{10} - 8} = {{\log_{10}\frac{\alpha}{2.5}} + {\log_{10}{Vb}} - {3\;\log_{10}D}}} & \left( {{Expression}\mspace{14mu} 29} \right) \\{\mspace{79mu}{{\left( {\beta + 30} \right)\log_{10}D} = {80 + {10\;\log_{10}\frac{\alpha}{2.5}} + {\log_{10}{Vb}} - \gamma}}} & \left( {{Expression}\mspace{14mu} 30} \right) \\{\mspace{79mu}{{\log_{10}D} = \frac{80 + {10\;\log_{10}\frac{\alpha}{2.5}} + {10\;\log_{10}{Vb}} - \gamma}{\beta + 30}}} & \left( {{Expression}\mspace{14mu} 31} \right) \\{D = {10^{\frac{80 + {10\;\log_{10}\frac{\alpha}{2.5}} + {10\;\log_{10}{Vb}} - \gamma}{\beta + 30}} = {Doffset}}} & \left( {{Expression}\mspace{14mu} 32} \right)\end{matrix}$

Expression 32 is equivalent to the formula for computation of offsetdistance. A value obtained by substituting the actually detected speedVb of the preceding vehicle into Expression 32 is used as the offsetdistance Doffset.

Expression 32 is computed from the brake discriminant indicated byExpression 25. The brake discriminant indicated by Expression 25indicates a distance at which the driver starts brake operation.Therefore, it can be said that Expression 32 (formula for computation ofoffset distance), which can be computed from the brake discriminant ofExpression 24, indicates the distance D to the preceding vehicle atwhich the driver starts brake operation. For this reason, the driver canget the feeling of safety from deceleration control when the distance Dto the preceding vehicle, computed using Expression 32 (formula forcomputation of offset distance) is taken as the offset distance Doffset.

Especially, adaptive cruise control is performed so that the relativespeed Vr becomes zero. The formula for computation of offset distance ofExpression 32 is obtained by zeroing the relative speed Vr. Whenadaptive cruise control is active, for this reason, it is especiallyfavorable to use the offset distance Doffset computed using the formulafor computation of offset distance of Expression 32.

The present invention is not limited to the above embodiments at all andcan be implemented in many modified ways.

For example, when the preceding vehicle rapidly decelerates or the roadcondition is bad and the subject vehicle cannot decelerate with a targetdeceleration, the subject vehicle cannot complete deceleration controlat the offset distance Doffset and further approaches the precedingvehicle.

To cope with this, for example, the following measure may be taken whenthe actual inter-vehicle distance D to the preceding vehicle becomesshorter than the offset distance Doffset: the target deceleration of thesubject vehicle when the actual inter-vehicle distance D to thepreceding vehicle is equal to the offset distance Doffset is maintainedand the execution of deceleration control is continued; or thedeceleration (Vr/t0) obtained by dividing the relative speed Vr betweenthe subject vehicle and the preceding vehicle by a predetermined time t0is taken as the acceleration/deceleration G_(Dp) of the subject vehicleto be taken as a target and the execution of deceleration control iscontinued.

When the actual inter-vehicle distance D becomes shorter than the offsetdistance Doffset, determination is performed using the following brakediscriminant: the brake discriminant KdBc (Expression 33, solid line inFIG. 4) in which the zero value of the actual inter-vehicle distance Dto the preceding vehicle is taken as the zero value of inter-vehicledistance in the brake discriminant KdBc, as illustrated in FIG. 14:KdBc=−23.76×log₁₀ D+76.96  (Expression 33)

Thus, even when the actual inter-vehicle distance D to the precedingvehicle becomes shorter than the offset distance Doffset, furtherapproach to the preceding vehicle can be suppressed.

1. A speed control system for vehicles comprising: means for detecting adistance to a preceding vehicle; means for detecting a relative speedbetween a subject vehicle and the preceding vehicle; means for computinga corrected distance condition evaluation index which is increased withincrease in the relative speed when approaching the preceding vehicleand whose increase gradient becomes greater as the distance to thepreceding vehicle becomes shorter at each relative speed as an indexindicating the condition of approaching or separating from the precedingvehicle by taking into consideration of a speed of the precedingvehicle; first means for setting a threshold value of the correcteddistance condition evaluation index and taking an offset amount presetwith respect to distance as zero value of distance at the thresholdvalue of the corrected distance condition evaluation index; means forchecking whether the corrected distance condition evaluation index ishigher than the threshold value of the corrected distance conditionevaluation index; second means for setting the threshold value of thecorrected distance condition evaluation index as a target value of thecorrected distance condition evaluation index when the correcteddistance condition evaluation index is higher than the threshold valueof the corrected distance; means for computing the target decelerationof the subject vehicle based on a target relative speed corresponding tothe target value set by the means for setting the threshold value as thetarget value and an actual relative speed; and means for startingdeceleration control to decelerate the subject vehicle so thatdeceleration of the subject vehicle becomes equal to the targetdeceleration.
 2. The speed control system for vehicles of claim 1,wherein the starting means maintains the target deceleration of thesubject vehicle when the actual distance is equal to the offset amountand continues the execution of deceleration control, when the actualdistance is shorter than the offset amount.
 3. The speed control systemfor vehicles of claim 1, wherein the starting means takes a decelerationobtained by dividing the relative speed by a predetermined time as thetarget deceleration of the subject vehicle and continues execution ofdeceleration control, when the actual distance is shorter than theoffset amount.
 4. The speed control system for vehicles of claim 1,wherein: the first setting means also computes the threshold value of acorrected distance condition evaluation index in which the zero value ofthe actual distance is taken as the zero value of distance at thethreshold value of the corrected distance condition evaluation index;and the checking means carries out determination using the thresholdvalue of the corrected distance condition evaluation index in which thezero value of the actual distance to the preceding vehicle is taken asthe zero value of distance at the threshold value of the correcteddistance condition evaluation index, when the actual distance is shorterthan the offset amount.
 5. The speed control system for vehicles ofclaim 1, wherein the offset amount is set based on speed control settinginformation of the subject vehicle.
 6. The speed control system forvehicles of claim 5, wherein, when the speed control setting informationindicates that control to carry out speed control so that the relativespeed between the subject vehicle and the preceding vehicle becomes zeroat a constant distance to the preceding vehicle is active, the offsetamount is set using the following formula for computation of offsetdistance $\begin{matrix}{{Doffset} = 10^{\frac{80 + {10\;\log_{10}\frac{\alpha}{2.5}} + {10\;\log_{10}{Vb}} - \gamma}{\beta + 30}}} & \;\end{matrix}$ where, α, β and γ are constants determined based onexperiment and Vb is the speed of the preceding vehicle.
 7. The speedcontrol system for vehicles of claim 1, wherein the offset amount is setbased on information acquired by wireless communication with an externalsource, the information being on external environment having influenceon traveling speed of the subject vehicle.
 8. A speed control system forvehicles comprising: means for detecting a distance to an object ahead;means for detecting a relative speed between a subject vehicle and theobject ahead; means for storing a correction formula for computation ofa corrected target relative speed obtained by correcting a formula forcomputation of a target relative speed with an offset amount indicatingthe target relative speed when the distance is zero, wherein the formulafor computation of target relative speed is determined from a distancecondition evaluation index relational expression and a tangentialexpression so that a target relative speed is determined based on thedistance, wherein, the distance condition evaluation index relationalexpression indicates a relation among the distance condition evaluationindex indicating condition of distance to the object ahead, a distanceto the object ahead and the relative speed, so that the distancecondition evaluation index is increased with increase in the relativespeed when approaching the object ahead and an increase gradient of thedistance condition evaluation index becomes greater as the distance tothe object ahead becomes shorter at identical relative speed, andwherein the tangential expression indicates a tangent of a curverepresented by the distance condition evaluation index relationalexpression, and is determined by differentiating the distance conditionevaluation index relational expression by the distance, therebyindicating a relation between the distance condition evaluation indexand the distance; means for computing a target relative speed from thecorrection formula for computation of target relative speed and adistance actually detected by the means for detecting the distance;means for computing a target deceleration from a target relative speedcomputed by the means for computing a target relative speed and arelative speed actually detected by the means for detecting the relativespeed; and means for performing deceleration control on the subjectvehicle based on the target deceleration.
 9. The speed control systemfor vehicles of claim 8, wherein the offset amount is set based on speedcontrol setting information of the subject vehicle.
 10. The speedcontrol system for vehicles of claim 9, wherein, when the speed controlsetting information indicates that control to carry out speed control sothat the relative speed between the subject vehicle and the precedingvehicle becomes zero at a constant distance to the preceding vehicle isactive, the offset amount is set using the following formula forcomputation of offset distance${Doffset} = 10^{\frac{80 + {10\;\log_{10}\frac{\alpha}{2.5}} + {10\;\log_{10}{Vb}} - \gamma}{\beta + 30}}$where, α, β and γ are constants determined based on experiment and Vb isthe speed of the preceding vehicle.
 11. The speed control system forvehicles of claim 8, wherein the offset amount is set based oninformation acquired by wireless communication with an external source,the information being on external environment having influence ontraveling speed of the subject vehicle.
 12. A speed control system forvehicles comprising: means for detecting a distance to an object ahead;means for detecting a relative speed between a subject vehicle and theobject ahead; means for storing a correction formula for computation ofa corrected target relative speed obtained by correcting a formula forcomputation of a target relative speed with an offset amount indicatingthe distance when the relative speed is zero, wherein the formula forcomputation of target relative speed is determined from a distancecondition evaluation index relational expression and a tangentialexpression so that a target relative speed is determined based on thedistance, wherein, the distance condition evaluation index relationalexpression indicates a relation among the distance condition evaluationindex indicating condition of distance to the object ahead, a distanceto the object ahead and the relative speed, so that the distancecondition evaluation index is increased with increase in the relativespeed when approaching the object ahead and an increase gradient of thedistance condition evaluation index becomes greater as the distance tothe object ahead becomes shorter at identical relative speed, andwherein the tangential expression indicates a tangent of a curverepresented by the distance condition evaluation index relationalexpression, and is determined by differentiating the distance conditionevaluation index relational expression by the distance, therebyindicating a relation between the distance condition evaluation indexand the distance; means for computing a target relative speed from thecorrection formula for computation of target relative speed and adistance actually detected by the means for detecting the distance;means for computing a target deceleration from a target relative speedcomputed by the means for computing a target relative speed and arelative speed actually detected by the means for detecting the relativespeed; and means for performing deceleration control on the subjectvehicle based on the target deceleration.
 13. The speed control systemfor vehicles of claim 12, wherein: the distance condition evaluationindex is computed as a corrected distance condition evaluation indexfrom the relative speed between the subject vehicle and the precedingvehicle, the speed of the preceding vehicle and the distance to thepreceding vehicle, so that the distance condition to the precedingvehicle in consideration of the speed of the preceding vehicle isindicated, the corrected distance condition evaluation index beingincreased with increase in the relative speed when approaching thepreceding vehicle, and an increase gradient becoming greater as thedistance to the preceding vehicle becomes shorter at each relativespeed; the offset amount in the formula for computation of correctedtarget relative speed uses the distance to the preceding vehiclecomputed by taking the speed of the preceding vehicle as actualdetection value in a formula for computation of offset distance, whichindicates a relation between the distance to the preceding vehicle andthe speed of the preceding vehicle, and which is obtained from a formulafor computing the corrected evaluation index and a deceleration startdiscriminant by zeroing the relative speed, the deceleration startdiscriminant indicating a relation between the corrected distancecondition evaluation index and the distance to the preceding vehiclewhen deceleration of the subject vehicle is started.
 14. The speedcontrol system for vehicles of claim 12, wherein the starting meansmaintains the target deceleration of the subject vehicle when the actualdistance is equal to the offset amount and continues the execution ofdeceleration control, when the actual distance is shorter than theoffset amount.
 15. The speed control system for vehicles of claim 12,wherein the starting means takes a deceleration obtained by dividing therelative speed by a predetermined time as the target deceleration of thesubject vehicle and continues execution of deceleration control, whenthe actual distance is shorter than the offset amount.
 16. The speedcontrol system for vehicles of claim 12, wherein the offset amount isset based on speed control setting information of the subject vehicle.17. The speed control system for vehicles of claim 16, wherein, when thespeed control setting information indicates that control to carry outspeed control so that the relative speed between the subject vehicle andthe preceding vehicle becomes zero at a constant distance to thepreceding vehicle is active, the offset amount is set using thefollowing formula for computation of offset distance${Doffset} = 10^{\frac{80 + {10\;\log_{10}\frac{\alpha}{2.5}} + {10\;\log_{10}{Vb}} - \gamma}{\beta + 30}}$where, α, β and γ are constants determined based on experiment and Vb isthe speed of the preceding vehicle.
 18. The speed control system forvehicles of claim 12, wherein the offset amount is set based oninformation acquired by wireless communication with an external source,the information being on external environment having influence ontraveling speed of the subject vehicle.